The Spotted alfalfa aphid occurs only as asexually reproducing females. They breed almost continuously throughout the year, but are most favoured by warm, dry weather. Under optimal conditions they are capable of rapid rates of reproduction. Over summer they feed on summer green legumes and lucerne. Winged aphids may migrate to annual medics and clovers in autumn.
Origin and History:
The Spotted alfalfa is a pest of lucerne, subterranean and strawberry clover and annual medics. They are most favoured by warm, dry weather. Outbreaks are usually confined to summer and early autumn.
Medics and clovers being winter growing annuals are damaged by Spotted alfalfa aphid either early, when germinating, or in spring if growth extends through to warm weather. Medic plants become stunted, yellow and sticky, eventually dying if under heavy aphid infestation. Yellow and dead patches appear within the sward.
Subterranean and strawberry clover seedlings under Spotted alfalfa aphid attack become stunted with outer leaves becoming mottled and reddish grey, while younger inner leaves become pale and yellow. Surviving plants can remain unhealthy and unproductive well into winter.
In lucerne Spotted alfalfa aphids congregate on the lower parts of the plant and spread to the stems as infestations increase. The leaves turn yellow, wilt and drop off progressively from the bottom of the plant. Heavy infestations left untreated can reduce lucerne stands to stalks. These become sticky with honey dew exuded by the aphids and then black as black sooty mould grows on the honey dew. Attempts to make hay from heavily infested lucerne stands have been unsatisfactory. Hay balers have been severely damaged because the sticky nature of the honey dew prevents free flow of material through the baler.
Management and Control:
Sowing of resistant or tolerant cultivars of lucerne, medic or clovers can play a major role in overcoming both problems.
Parasitic wasps, hover flies, lady birds, lace wings and fungal disease are biocontrol agents. Biocontrol agents are only effective when aphid numbers are low.
Under optimum conditions where aphids have the ability to rapidly multiply the use of insecticides maybe necessary. There is a number of insecticides available for Spotted alfalfa aphid control. Use of chemicals that are solely aphicides and 'soft' of beneficial insects in important to encourage biocontrol agents.
Collated by HerbiGuide. Phone 08 98444064 for more information.